1. Field of the Invention
The present invention relates to calibration fixtures and more particularly to a calibration fixture used in calibrating a current probe.
2. Description of the Prior Art
Current probes are used to apply magnetic fields to a conductor or conductors which exist between electrical components being tested. This is to simulate the magnetic fields which might be encountered when the equipment is subjected to an RF energy environment in actual use. For example, aircraft components may be subjected to large RF energy fields when they pass through radio, television or radar transmissions. The government has set up certain environmental conditions which manufacturers must use to test equipment being sold. One of such conditions may be that the equipment must operate satisfactorily in RF fields which generate no more than one amp of current in the conductors joining two electronic components. The governmental conditions may be expressed in a requirement that when exposed to RF energy in a range between two frequencies, a current of no more than predetermined magnitude will be induced in a conductor of predetermined impedance. One example of the required conditions is found in a document "RTCA/DO-160C ENVIRONMENTAL CONDITIONS AND TEST PROCEDURES FOR AIRBORNE EQUIPMENT". The requirements here are that no more than 300 milliamps be induced in a 50 ohm conductor over a 10 kHz to 400 MHz range. In order to assure that equipment is tested under these conditions, it is necessary to have a device that causes the current induction over the desired range. A current probe consisting of conductive coils enclosed in an e field shield or Faraday shield (a metal housing to contain the fields generated and shaped to surround the conductor or conductors) has been used for this purpose but it is necessary to make sure that the current probe is itself properly calibrated in order to know how much current it will induce at various frequencies. In the prior art, this has been done with a fixture which includes a central conductor passing through the center of the probe and an RF generator to supply the surrounding coils of the probe with varying amounts of energy at various frequencies and to induce a current in the conductor. In the example of the aircraft equipment above, the coils of the current probe would be supplied with power from an RF power generator and amplifier at frequencies from 10 kHz to 400 MHz at levels necessary to induce no more than 300 milliamps of current. Calibration fixtures available in the prior art comprise a fixture which holds a conductor in the central opening of the probe. The external connectors to the conductor are designed to be of 50 ohm impedance but the center conductor is not. As a result, the fixture surrounding the probe coacts with the probe and the center conductor to produce impedances which were very low at certain frequencies and this produces excessive currents. In some cases, using the prior art test fixtures, we have encountered currents high enough to destroy the equipment.